When using the New Relic REST API, you need to replace some ID placeholders (like $APP_ID, $KEYTX_ID, $COMPONENT_ID, etc.) with the ID for an app, transaction, or other component.
You can use the New Relic UI to get some of these IDs. You can also get a list of IDs using the REST API Explorer.
Find IDs using the UI
The UI includes the IDs of applications and some other components in a metadata panel. To see an entity's metadata, click on a monitored entity from the entity explorer list, and click the tag icon.